R: how does a foreach loop find a function that should be invoked?

Question

I have problems when I use a foreach loop (using %dopar%) which invokes a self-defined function. There is not really a problem when I work with Linux, but when I use Windows the self-defined function cannot be found. It is hard to explain the problem in words, so I composed a small example to show it. Assume I have a collection of three simple functions, where FUN2 (using %do%) and FUN3 (using %dopar%) invoke the first one (FUN):

FUN <- function(x,y,z) { x + y + z }
FUN2 <- function(a, b) {
  foreach(i=1:3) %do% FUN(i, a, b)
}
FUN3 <- function(a, b) {
  foreach(i=1:3) %dopar% FUN(i, a, b)
}

The functions are stored in a script called foreach_testfunctions.R. In another script (foreach.test) I source these functions, use library(doParallel) and try to use the functions. First I do it with Linux and all works fine:

source("foreach_testfunctions.R")
a <- 2
b <- 3
library(doParallel)
registerDoParallel()

foreach(i=1:3) %do% FUN(i, a, b)    ## works fine
FUN2(a, b)                          ## works fine
foreach(i=1:3) %dopar% FUN(i, a, b) ## works fine
FUN3(a, b)                          ## works fine 

Then I do it in Windows:

source("foreach_testfunctions.R")
a <- 2
b <- 3
library(doParallel)
cl <- makeCluster(3)
registerDoParallel(cl)

foreach(i=1:3) %do% FUN(i, a, b)    ## works fine
FUN2(a, b)                          ## works fine
foreach(i=1:3) %dopar% FUN(i, a, b) ## works fine
FUN3(a, b)                          ## does not work
Error in FUN(i, a, b) : task 1 failed - "Could not find function "FUN""

Conclusion: (1) No problems with %do%. (2) Problems with %dopar% when using Windows. I tried inserting the line clusterExport(cl, varlist=c("FUN", "a", "b"), env=environment()) before the line that invokes FUN3 to make sure that the function FUN and the variables a and b are found in the proper environment, but the error remains.

My questions: Why does Windows behave different than Linux although the code is identical (apart from the different registerDoParallel syntax)? How can I make sure that Windows does find function FUN when invoked via function FUN3?

Answer 1

They behave differently because registerDoParallel registers an mclapply backend on Linux, while it registers a clusterApplyLB backend on Windows. When using an mclapply backend, there are essentially no data exporting issues, so it works on Linux. But with clusterApplyLB, you can run into problems if foreach doesn't auto-export the functions and data that are needed.

You can solve this problem by modifying FUN3 to export FUN via the .export option:

FUN3 <- function(a, b) {
  foreach(i=1:3, .export='FUN') %dopar% FUN(i, a, b)
}

This solution works on both Linux and Windows, since .export is ignored by the mclapply backend.

As pointed out by Hong Ooi, you have an error in your use of clusterExport, but I wouldn't use clusterExport to solve the problem since it is backend specific.